1. Field of the Invention
The present invention relates to a multi-frequency signal receiver responsive to multi-frequency signaling in an audio-frequency band for transmitting multiple pieces of information in the form of multi-frequency tone signals with the use of an audio-frequency band, and more particularly to a multi-frequency signal receiver which is suitable for receiving push button dial signals (which will be referred to herein as "PB signals").
2. Description of the Prior Art
The PB signals transmitted from a telephone of the push-button type have the digits 0 to 9 and several symbols, each of which correspond to one of the combinations in frequencies between the low frequency group including 697 HZ, 770 Hz, 852 Hz and 941 Hz and the high frequency group including 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz.
In a time-division-multiplexed switching exchanger using pulse code modulation (PCM) as signals, the PB signals of such kind are converted to digital signals, which are transmitted to the receiver through a switching network and each time-division channel. As a receiving system for receiving the PB signals thus converted, there have conventionally been proposed (1) a counter method for detecting the signal frequencies from the measurements of the time intervals between alternate zero-crossings of the input waveform, (2) a PARCOR method for extracting the partial correlation coefficients of an input speech signal to analyze and (3) process the same, and a digital filter method, in which the construction of an analog type receiver is replaced by a digital one.
In case the multi-frequency receiver is realized using the above methods, the most important point to be considered is concerned with accuracy and economy. If the foregoing methods are examined from this standpoint, it is impossible for the counter method to attain the desired accuracy with a sampling frequency of 8 KHz for the PCM. If the observation period is elongated to enhance the accuracy, there arises a problem in that the observation period exceeds a predetermined signal continuation period.
According to the PARCOR method, on the other hand, the processing becomes so complex as to complicate the construction necessary for the processing with the resultant problem in the economy that the cost for the receiver per line becomes high.
According to the digital filter method, however, the desired accuracy can be attained for the pulse-code-modulated signals having a sampling frequency of 8 KHz. Thus, the economical realization of the digital filter method has heretofore been desired.
According to the digital filter method of the conventional type, the digitally coded PB signals having been sampled at the sampling frequency of 8 KHz are separated into two high and low frequency groups with the use of high and low frequency elimination filters, and these two frequency groups are supplied through limiters to band pass filters for the respective PB signal frequencies so that the signal frequencies may be detected. Since, in this conventional method, the digital signals of the sampling frequency of 8 KHz are processed as they are, there arises a problem in that the multiple signal processing capacity can not assume such a high level with the result that the receiver cost for each line becomes relatively high.